Mass spectrometers can measure mass-to-charge ratios m/z of ions originating from a compound, where the value of mass-to-charge ratios fluctuate due to various factors. The width of fluctuation of the measured values of mass-to-charge ratio is regarded as the mass accuracy of a given mass spectrometer. To enhance the mass accuracy, a mass calibration is normally performed for the mass spectrometer using measurement results of a compound whose theoretical value (or highly accurate measurement value) of the mass-to-charge ratio is known.
For example, apparatuses described in Patent Literature 1 and the like measure a standard sample containing a certain compound whose theoretical value of mass-to-charge ratio is known, compare an actual measured value and the theoretical value of the mass-to-charge ratio, and thereby determine a mass deviation at the mass-to-charge ratio. Then, based on mass deviations obtained at different mass-to-charge ratios of plural compounds, a calibration curve which represents a relationship between the mass-to-charge ratio and mass deviation is created. Based on the calibration curve thus created, the actual measured value of the mass-to-charge ratio obtained by measuring any compound in a target sample is calibrated. Such mass calibration allows the mass-to-charge ratio of a desired compound to be determined at high accuracy.
The mass calibration method described above measures the standard sample and target sample separately, and consequently it is not possible to eliminate mass deviations caused by differences in measurement conditions, environmental conditions, and the like used for measurements of the two samples. Another type of mass calibration is also performed using an internal standard method, when a peak originating from a known compound whose theoretical value of mass-to-charge ratio is known exists in a mass spectrum obtained by measuring a target sample. In the internal standard method, a mass deviation is determined using the actual measured value and theoretical value of the mass-to-charge ratio at the peak, and corrects the mass-to-charge ratios at other peaks in the mass spectrum based on the mass deviation. This mass calibration method performs mass calibration based on the results of measurement performed at a time, and thus the mass calibration is made at higher accuracy.
However, mass calibration of the internal standard method described above can be made only when a peak originating from a known compound exists in an acquired mass spectrum and can be detected.
In MSn spectra obtained by an ion trap time-of-flight mass spectrometer or by a tandem quadrupole mass spectrometer, various product ions produced by dissociation of a single compound selected based on a mass-to-charge ratio are observed, but, other than these product ions, an ion peak of a compound whose accurate mass-to-charge ratio is known does not exist in many cases. In such cases, mass calibration by the internal standard method described above cannot be used. Thus, conventionally it is common practice to perform mass calibration of the peaks of an MSn spectrum using mass deviation values or a mass calibration table obtained by the internal standard method on the MS1 spectrum (mass spectrum) obtained from the same sample without a dissociation operation (see Patent Literature 2 and the like). Consequently, it is unavoidable that the mass accuracy of an MSn spectrum is inferior to the mass accuracy of the MS1 spectrum.